1. Field of the Invention
The present invention relates generally to an analog to digital converting system for a video signal and is directed more particularly to an analog to digital converting system for a composite video signal.
2. Description of the Prior Art
When a composite video signal is analog to digital (A/D) converted, its sampling point must be the same position (phase) for a synchronizing pulse and a color subcarrier during any horizontal period. Therefore, in the prior art three points in one cycle of a burst signal are sampled and it is checked whether the three points are scheduled sampling points or not by the level of the sampled output. Then, based upon the checked result the sampling phase for the composite video signal is controlled.
According to the above method, however, since the number of the sampling outputs is small, the produced control signal contains much uncertain portions and also an error is large. Further, this method is effective for only such a case that the sampling frequency is an integer multiple of the color subcarrier frequency (burst frequency). For example, when the sampling frequency is 13.5 MHz which is not an integer multiple, an error is caused and hence the control for the sampling phase becomes unstable.